Exploring the Likelihood and Mechanism of a Climate-Change-Induced Dieback of the Amazon Rainforest
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Exploring the likelihood and mechanism of a climate-change-induced dieback of the Amazon rainforest Yadvinder Malhia,1, Luiz E. O. C. Araga˜ oa, David Galbraithb, Chris Huntingfordc, Rosie Fisherd, Przemyslaw Zelazowskia, Stephen Sitche, Carol McSweeneya, and Patrick Meirb aEnvironmental Change Institute, School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, United Kingdom; bSchool of GeoSciences, University of Edinburgh, Edinburgh EH8 9XP, United Kingdom; cCentre for Ecology and Hydrology, Wallingford OX10 8BB, United Kingdom; dDepartment of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom; and eMet Office Hadley Centre, Joint Centre for Hydro-Meteorological Research, Wallingford OX10 8BB, United Kingdom Edited by Hans Joachim Schellnhuber, Potsdam Institute for Climate Impact Research, Potsdam, Germany, and approved December 15, 2008 (received for review June 10, 2008) We examine the evidence for the possibility that 21st-century nonlinearities, thresholds, and feedbacks and determine which climate change may cause a large-scale ‘‘dieback’’ or degradation components of this system are open to manipulation and man- of Amazonian rainforest. We employ a new framework for eval- agement in a manner beneficial to the long-term sustainability of uating the rainfall regime of tropical forests and from this deduce the Amazonian social ecological system. precipitation-based boundaries for current forest viability. We In this paper, we ask 3 questions. First, what are the climatic then examine climate simulations by 19 global climate models thresholds that favor the current presence of a forest as opposed (GCMs) in this context and find that most tend to underestimate to savanna? Second, what do climate models say about the likely current rainfall. GCMs also vary greatly in their projections of direction of expected climate change in Amazonia and any future climate change in Amazonia. We attempt to take into associated likelihood of large regions of Amazonia crossing account the differences between GCM-simulated and observed thresholds of forest viability during the 21st century? Third, to rainfall regimes in the 20th century. Our analysis suggests that what extent does direct human pressure (deforestation, frag- dry-season water stress is likely to increase in E. Amazonia over the mentation, and fires) influence the transition? 21st century, but the region tends toward a climate more appro- priate to seasonal forest than to savanna. These seasonal forests Current Climate and Vegetation in Amazonia may be resilient to seasonal drought but are likely to face inten- The lowland forests of Amazonia have a mean annual temper- sified water stress caused by higher temperatures and to be ature of 26 °C, with very little spatial variability, and a mean vulnerable to fires, which are at present naturally rare in much of annual precipitation of Ϸ2400 mm, ranging from Ͼ3000 mm in Amazonia. The spread of fire ignition associated with advancing North West Amazonia to Ͻ1500 mm at the forest–savanna deforestation, logging, and fragmentation may act as nucleation transition zones (5). points that trigger the transition of these seasonal forests into Two relevant features of the rainfall regime are (i) the fire-dominated, low biomass forests. Conversely, deliberate limi- intensity and duration of the dry season and (ii) the overall water tation of deforestation and fire may be an effective intervention to supply. To describe the accumulated water stress that occurs maintain Amazonian forest resilience in the face of imposed across a dry season, we employ the maximum climatological 21st-century climate change. Such intervention may be enough to water deficit (MCWD) (6). navigate E. Amazonia away from a possible ‘‘tipping point,’’ MCWD is defined as the most negative value of climatological beyond which extensive rainforest would become unsustainable. water deficit (CWD), attained over a year, where the monthly change in water deficit is precipitation (P) (mm/month) Ϫ ͉ ͉ ͉ ͉ carbon dioxide drought fire tropical forests adaptation evapotranspiration (E) (mm/month). For month n, ϭ ϩ Ϫ ͑ ͒ ϭ he response of components of the Earth system to increasing CWDn CWDnϪ1 Pn En; Max CWDn 0; levels of anthropogenic greenhouse-gas forcing is unlikely to T CWD ϭ CWD ; MCWD ϭ Min͑CWD ... CWD ͒ be continuous and gradual; instead there may be ‘‘tipping 0 12 1 12 elements’’ in the system (1). Among the most iconic of these is At the wettest time of the year, we assume the soil is saturated (i.e., the Amazon rainforest, with some projections suggesting the set CWD ϭ 0) and start the 12-month cycle of calculation from this possibility of substantial and rapid ‘‘dieback’’ (2–4). The Ama- wet phase. For any multiyear period, we apply this calculation to the zon forest biome is biologically the richest region on Earth, Ϸ mean annual cycle of precipitation (rather than calculating for each hosting 25% of global biodiversity, and is a major contributor year and then taking the mean MCWD). We do not attempt to to the biogeochemical functioning of the Earth system (3). Its model E but fix it at 3.33 mm/day, Ϸ100 mm/month. Hence, the large-scale degradation would leave an enduring legacy on the CWD is only an approximate indicator of actual soil water deficit functioning and diversity of the biosphere. We review the evidence for such a tipping element in Amazonia and examine climate model projections in the context of rainforest viability Author contributions: Y.M. designed research; Y.M., L.E.O.C.A., R.F., P.Z., and S.S. per- considering direct human pressures on the forest system. formed research; Y.M., L.E.O.C.A., D.G., R.F., P.Z., and C.M. analyzed data; and Y.M., C.H., There is clear and ongoing change in the physical environment S.S., and P.M. wrote the paper. of Amazonia, whether through increasing atmospheric carbon The authors declare no conflict of interest. dioxide concentrations, associated imposed climate change, or This article is a PNAS Direct Submission. more direct intervention because of the spread of settlement, 1To whom correspondence should be addressed. E-mail: [email protected]. deforestation, forest timber extraction, or any related fire initi- This article contains supporting information online at www.pnas.org/cgi/content/full/ ation. Such perturbations are certain to persist on policy-relevant 0804619106/DCSupplemental. timescales. The challenge is to identify and characterize system © 2009 by The National Academy of Sciences of the USA 20610–20615 ͉ PNAS ͉ December 8, 2009 ͉ vol. 106 ͉ no. 49 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0804619106 Downloaded by guest on September 26, 2021 rainfall regime space defined by AP and MCWD. The analysis focuses on an area between 45°W and 70°W and 0° and 20°S, SPECIAL FEATURE covering both forest and savanna. It is evident that there are no sharp vegetation thresholds in the rainfall regime space (MCWD, AP); some savanna pixels are found in predominantly forest climates, and some evergreen forests are found in dry climates. The diffuseness of the boundary reflects variation in local surface hydrology and soil properties, e.g., seasonal flooding can favor savanna in dry forest climates, shallow water tables can allow gallery or riverine forest to persist in dry savanna climates, or more fertile soil may favor trees over grasses. The fuzziness in the thresholds may also reflect errors in vegetation or phenological classification or in the rainfall map. Nevertheless, it is possible to identify broad climatic thresholds consistent with the definition of tipping points. Evergreen forests tend to predominate if the dry season is weak (MCWD ϾϪ200 mm). If AP Ͼ 1500 mm, forests tend to predominate, intuitively becoming increasingly seasonal and decid- uous for more negative values of MCWD (the land-use classifica- Fig. 1. The relationship between vegetation type and rainfall regime. tion cannot distinguish evergreen from semideciduous forests). If Rainfall regime is derived from TRMM for the period 1998–2006. Our sug- Ͻ ϽϪ gested savanna zone is shaded (MCWD ϽϪ300 mm, AP Ͻ 1500 mm). AP 1500 mm, savannas predominate if MCWD 400 mm, and there is a broad transition zone for Ϫ400 mm Ͻ MCWD ϽϪ200 mm, where there is a gradual shift in the relative abundance of because it does not account for seasonal variation in E (driven savanna relative to forest. For analyses later in this paper, we ascribe mainly by radiation and phenology) (7) or spatial variation in E the following terms to bioclimatic spaces (not necessarily the related to soil and root properties. In addition, actual transpiration vegetation types): (i) ‘‘rainforest’’: MCWD ϾϪ200 mm; (ii) rates may increase with rising temperatures; the limitations of our ‘‘seasonal forest’’: MCWD ϽϪ200 mm and rainfall Ͼ1500 mm (the assumption of fixed E are discussed in Influences of Temperature and transition between rainforest and seasonal forest climates is in ϽϪ CO2 Change. A fixed E is also inappropriate outside of lowland reality a continuum); (iii) ‘‘savanna’’: MCWD 300 mm (mid- tropical regions, where lower energy supply may result in lower E. point of the transition range) and rainfall Ͻϭ1500 mm (latter As a metric of overall water supply, we utilize annual precipita- shaded in Fig. 1). tion (AP) (mm); for a given MCWD, higher values of AP reflect Climate and Atmospheric Change in Amazonia higher rainfall during the wet season. AP and MCWD are mapped in Fig. S1 A and B. For spatial analysis, these data are derived from Recently, temperatures in lowland tropical regions worldwide Ϸ NASA’s Tropical Rainfall Monitoring Mission (TRMM) satellite have been increasing at 0.25 °C per decade (5) and are for the period 1998–2005. TRMM has the advantage of complete projected to rise by 3–8 °C (mean 5 °C) over the 21st century coverage across otherwise data-poor areas, although it may under- under the A2 emissions scenario (Fig.